One of the major problems for radio communication systems providing wide area two-way data and/or voice communication coverage is how to allow for a communication unit operating in the system to roam freely between sub-systems which form the wide area system. For example, a 10 site simulcast radio frequency trunked system may be considered a wide area two-way system on a regional level. The problem with such systems is how to provide a way of allowing the communication unit which is part of the system the ability to roam freely so that it is always part of a larger composite wide area system (or communication network), thereby allowing a caller to be able to communicate with the communication unit at any time the communication unit is in range of the wide area system.
Present day solutions to the above problem include communication systems such as Coverage Plus.TM. manufactured by Motorola, Inc. This system uses a registration process in which the roaming communications units use a "checking in" process on each communication network site as the communication unit moves from site to site. This "check-in" process occurs even if the communication unit is not communicating with other units.
In wide area systems, knowing the location of each of the communication units "ahead of time", before a call request that will involve these units occurs, helps speed up group calls that involve activating channel resources at many sites across the wide area system. Present day registration processes take a lot of time on the site control channel which has the effect of limiting the overall capacity of the local site. The overall effect on system capacity will depend on the number of roaming units and the frequency of operation the units are employing when moving from coverage area to coverage area (i.e., site to site registration "hand-offs"). Currently this is not a major problem with the current numbers of roaming units and the relatively large size of the coverage areas, since typically larger coverage areas see fewer hand-offs. However, as systems become more complex, and Coverage areas become smaller in size, system overhead delays will become a major problem for wide area systems.
In wide area trunked systems, communication units must find a site within their present coverage area by either scanning through a list of pre-programmed control channels or they must scan the entire trunking band for a "friendly" control channel that is broadcasting a "networking offered here" outbound signalling word (OSW). In other words, the communication unit must search for a control channel which is compatible with the communication unit. As one can quickly determine, acquiring access to a local site, while roaming into unknown coverage areas, in the manner discussed above, is extremely time consuming.
In comparison to wide area trunked systems, cellular telephone systems have relatively small cells and large numbers of communication unit users that only do individual oriented calls (i.e., mobile-to-land and land-to-mobile telephone calls). Given this, cellular systems do not use unit-to-site registration processes. Due to the relative low probability of an infrequent incoming land-to-mobile call, it is better for cellular systems to just send a unit respond signal over the control channel from every cell when incoming calls actually occur. Speed of call set ups is not of utmost criticality for these one unit phone calls. Therefore, the unit respond signals can go out constantly, over a period of several seconds, in order to determine if the communication unit is turned on and in communication range. Each cellular radio must scan the different system control channels as the unit moves from cell to cell.
In nation-wide satellite paging systems, although providing wide area coverage, these systems do not direct roaming communication units to local two-way systems. Also, these paging systems do not provide the pagers with local system availability information in order for the pagers to receive further information once they have been paged.
Finally, system to system roaming, like that implemented by Ameritech Inc. in their "Easy Roaming" scheme, use a roaming unit-to-system registration process as the communication unit enters a non-home system. In this system, the registration process sends a message back to the communication unit's home system so it knows where to forward future calls addressed to the unit, should someone try to reach the roaming unit at its normal home system number. One problem with this roaming arrangement is that calls can only be completed if the unit-to-location information is constantly known. This "back to the home system" registration process suffers when communication units are not able to reach the home system (e.g., RF noise, traffic, etc.). This type of registration process also tends to add a lot of control traffic overhead that may be unnecessary to the wide area system.
More importantly than all of the above mentioned problems is that neither cellular or trunked communication systems are not presently located everywhere, in order to allow communication units the freedom to roam over wide coverage areas. Presently, communication units are not constructed such that they can use either system type interchangeably. For example, today's cellular radios can't switch over to trunking when a cellular system is not available and a trunked system is, or vice versa. The problem has not been limited by the technology of a radio that has sufficient RF bandwidth to cover both services. The problem arises in that the roaming communication units can't decide which system to move to, even if they could, since the communication units do not know how to find these different types of systems quickly enough and they also do not know which one of the communication systems is "best" for their particular service needs at any given point in time (e.g., data, voice, etc.). This scenario will only worsen as newer small cell systems and other different types of communication systems emerge. With more different communication systems become available to effectuate a wide area communication system. As can be seen from the above discussion of prior art systems, a need exists for a way of controlling communication units which operate over wide areas encompassing more than one communication system.